1. Field of the Invention
The present invention relates to a radio communication system using a spread spectrum scheme and, more specifically, to an improvement of a spread spectrum radio communication system used in, for example, a radio LAN (local area network), and a mobile communication system.
2. Description of the Prior Art
In recent years, the spread spectrum scheme attracts attention for use in a radio communication system and as a communication means using very weak electromagnetic waves. Since the spread spectrum scheme has inherent resistance to multipath interference and allows multiple access within the limited bandwidths available, it is expected to play an important role in the field of mobile communications and premise radio communications. Several spread spectrum schemes have been known including the direct-sequence (DS) scheme, the frequency hopping (FH) scheme and a combination of those schemes.
FIG. 1 shows a transmission side configuration of the DS scheme. A serial data signal is converted to a k-bit parallel data signal by a serial-to-parallel converter 1, and the parallel data signal is modulated or encoded by a first modulator 2. The output signal of the first modulator 2 is then multiplied in a multiplier 4 by a spreading code that is supplied from a spreading code generator 3. The signal thus processed is transmitted through a radio transmitter 5 and an antenna 6.
In the above operation, a relationship
W=2URb, Ra=mRbxe2x80x83xe2x80x83(1)
is obtained where Ra (bits/sec) is a data rate of the serial data signal, Rb (symbols/sec) is a symbol rate of the output signal of the first modulator, W (Hz) is a bandwidth of the spectrum-spread signal, U is a spectrum spreading ratio, and m is a multi-value index of the first modulator 2.
From Equation (1), the data rate Ra of the serial data signal is expressed as
Ra=mW/2uxe2x80x83xe2x80x83(2)
In a premise radio communication system such as a wireless LAN, the serial data signal is required to be transmitted at high speed to secure matching with a cable LAN. As is understood from Equation (2), the following methods are conceivable to increase the transmission speed of the serial data signal.
a) Increase the spread bandwidth W.
b) Decrease the spectrum spreading ratio U.
c) Increase the number of the multi-value index m of the first modulation.
Usually, there exist some limitations on methods a) and b). For example, in the case of an ISM (industrial, scientific and medical) band radio LAN in Japan, there are limitations of bandwidth Wxe2x89xa626 MHz and spreading ratio Uxe2x89xa710. Therefore, to increase the transmission speed, method c) should be taken. Conventionally, QuadriPhase-Shift Keying (QPSK) (m=2) is employed as the first modulation in the DS system. To increase the transmission speed, the multi-value index m of the first modulation should be increased by employing 8PSK (1.5-fold) or 16PSK (2-fold). But in such cases, since distances between constellation signal points are reduced, the bit error rate increases in proportion to the multi-value index m, provided the signal-to-noise ratio (S/N) is kept the same.
An object of the present invention is to provide a radio communication system which enables high-speed data communication under limitations on frequency bands available and a spectrum spreading ratio.
Another object of the present invention is to provide a radio communication system which enables high-speed data communication without increasing the bit error rate.
Still another object of the invention is to provide a radio data transmitter and receiver which enable high-speed data communication in a radio communication system in which there are limitations on a frequency band used and a spectrum spreading ratio.
In the transmission side of a radio data communication system according to the invention, a k-bit parallel signal is divided into n signals each having k/n bits, which are encoded and then given a phase difference of xcfx80/2n between any two arbitrary consecutively encoded signals. Spectrum spreading is performed on the respective phase-shifted signals and then the spectrum spread signals are combined together to produce a transmission digital signal. In the receiving side, reverse spreading is performed on a receiving signal to produce n reverse-spread signals, and the reverse-spread signals are given the phase difference of xcfx80/2n to cancel the phase shift of the transmission side. The respective phase-shifted signals are demodulated or decoded to reproduce the original k-bit parallel signal.
More specifically, according to the invention, the transmission side of the radio data communication system comprises: a converter for converting a data signal to be transmitted to a k-bit parallel signal; encoders for mapping each of k/n-bit signals to signal point locations of a predetermined modulation scheme such as QPSK, the k/n-bit signals being produced by dividing the k-bit parallel signal into n parts; phase shifters for causing a phase difference of a multiple of xcfx80/2n between any two arbitrarily encoded signals adjacent to one other among the n encoded signals; spectrum spreading circuits for spreading each of the n phase-shifted encoded signals; an adder for combining the n spectrum spread signals to generate a digital transmission signal; a radio transmitter for transmitting a transmission wave by performing, for instance, quadrature modulation based on the digital transmission signal.
According to the invention, the receiving side of the radio data communication system comprises a radio receiver for demodulating a receiving wave by, for instance, quadrature demodulation to produce a digital receiving signal; spectrum reverse-spreading circuits for reverse-spreading the digital receiving signal to produce n reverse-spread digital signals; phase shifters for causing a phase difference of xcfx80/2n for cancelling the phase shift of the transmission side; and decoders for decoding each of the n phase-shifted digital signals according to a demodulation scheme corresponding to the predetermined modulation scheme, to thereby reproduce the k-bit parallel signal.
Therefore, according to the radio data communication system of the invention, the data transmission speed can be increased without increasing the multi-value index m in the first modulation. It is sufficient to use a QPSK encoder for a first modulation. Therefore, the high-speed transmission can be achieved without increasing the bit error rate.